1. Field of the Invention
The present invention relates to a mobile communication system and a transmission power control method for a base station in the mobile communication system, in particular, to those applicable to a Code Division Multiple Access (hereinafter, called as CDMA) communication scheme, effectively.
2. Description of the Related Art
In a mobile communication system, a mobile station is connected with a base station to which a transmission power can be minimized. The base station to which a connection request is issued by the mobile station communicates with the requesting mobile station by controlling its transmission power such that a received power from the mobile station comes to a coincide with that preset by the base station in advance.
On the above described mobile communication system, a reverse link, mobile to base communication, in a mobile communication system based on a CDMA communication scheme will create a near-far problem: received signals from transmitters close to the receiver are strong, while those from far transmitters are weak. Accordingly, a signal to interference power ratio (hereinafter, called as SIR) of a signal received from a mobile station far from the base station becomes small, which leads to degraded communication quality. In order to overcome the above described near-far problem, the transmission power of the mobile station is controlled such that the transmission signal power of the mobile station far from the base station increases whereas that of the mobile station near the base station decreases. This technology has been disclosed, for example, in an article entitled "Effects of Power Control Error on the System User Capacity of DS/CDMA Cellular Mobile Radios" authored by Eisuke Kudo and Tadashi Matsumoto, IEICE TRANS. COMMUN., Vol. E75-B, No. 6, June, 1992. In this article, a technology to solve the near-far problem is disclosed that the transmission signal power of the mobile station which are supervised by each base station is controlled by each base station such that the received signal power of each base station comes to coincide with a predetermined preset value: that is, the received power preset value becomes 1.0. By using the above technique, each base station controls SIR of the signal from the controlled mobile stations to have a constant value. In other words, since all communications in the CDMA scheme are performed within the same frequency band, SIR of the signal from a certain mobile station in the base station can be represented as follows: EQU SIR=P/((M-1).times.P+N) (1)
where M is a number of mobile stations connected with the base station; N is a sum of interference noises from mobile stations connected with the other base stations.
As mentioned above, each base station controls SIR from mobile stations to have a constant value. Accordingly, the transmission power control method described in the above article creates a problem that the values of SIR in each base station have deviations with each other when there is a non-uniform user distribution within the radio service area. In particular, assuming that the received power P has a constant value in accordance with the above formula (1), SIR depends on the number M of mobile stations and the interference noise N. Assuming the interference noise N is constant, SIR of the base station depends on the connection number M of mobile stations. In other words, assuming the interference noise N equals to 0; the number M of mobile stations connected with the base station A equals to 101 and the number M of mobile stations connected with the base station B equals to 11, then, SIRA of the base station A equals to 1/100, and SIRB of the base station B equals to 1/10. Accordingly, the more increases the number of connections with the base stations, the less SIR becomes, whereas the less the number of connections, the more SIR becomes.
As described above, if there is a non-uniform user distribution within the radio service area provided by the mobile communication system, the base station which has a dense user distribution of mobile stations reveals smaller SIR, which leads to degraded communication quality.
On the other hand, the base station which has a surplus user distribution of mobile stations reveals larger SIR, which leads to excessive communication quality. Accordingly, a non-uniform user distribution within the radio service area causes deviation of SIR in each base stations, which leads to degraded efficiency of the system as a whole.